99% of the time I just begin with wherever the seating die is from the last load as it really doesn't matter where you start from. The groups will either be good as is, 2+1, or a large triangle.
The reason being that the OAL maximum for each bullet will change anyway because of the ogive differences with each bullet so most of my OAL's tend to be not factory maximums nor anywhere near that length. The first groups are just to determine where you are starting from so in a way, this does not even rely on the typical maximum powder charges that people seek.
Remember that bench rest shooters don't care about velocity, only accuracy wins matches.
The other point here is that everything contributors have said here about variables over the years still has relevance to this point, beginning with powder lot variations, moisture losses with frequently opened powder cans, bullet variances/changes, primer variances, barrel and throat wear, length, maker, twist etc everything you ever read that is important to us, is a "so what, incidental" to a bench rest shooter. They don't care because they know they can never rely on the load from last match to win under new conditions which is why they walk around with trays of primed cases and set up a powder thrower.
The goal is accuracy that day, with what is on hand, regardless or historical variances and synergies. It is all about tuning a load to win the match at hand. That means where the seating die is set, the powder in the hopper, the primed cases prepped and start there. Once at the range is it what you have and tweak the seating die. The fact that the loads are charged and seated that day just adds to the uniformity of bullet tension on the case neck again, for that day.
The Aussie market was never as wide as the US market, gun shops were much smaller by comparison and consistent availability of the bullet you used last time almost never occurred so you tended to try a huge range of bullets because of that lack of consistent availability. By trying many loads over a lot of years, this tended to qualify the seating depth strategy over a wide range of rifles and chamberings.
I hope this works for many of you as I don't see too many rifles that don't respond to some degree, some dramatically.
John
